Possibly my favourite Florida cave. This video is from our January trip. We went back again in May for a trip through Rocky Horror and up through The Courtyard. I will post a video of that amazing dive soon. It is a beautiful cave.

In my last post (Choosing Trimix Diluent) I discussed one way of calculating and choosing your CCR diluent for a trimix dive. The process used did not consider O2 to be narcotic but how would you calculate the mix if you prefer to consider oxygen to be narcotic.

Our start point is exactly the same, we would choose the fraction of oxygen (FO2), based on a safe maximum PO2 of the diluent at our Maximum Operating Depth (MOD).

So using the same depth as the previous post as an example, on a dive to 70m and a maximum PO2 of our diluent at our MOD of 1.0 we would have a FO2 of: -

70m = 8 Bar

PO2/P (where P is the depth as an absolute pressure) = FO2

1.0/8 = 0.125

So we would probably choose an FO2 of 0.12 (12% O2) for the oxygen in our diluent.

Having calculated the FO2 we want, the next step is to calculate how much helium we will need in the loop to meet our chosen Equivalent Narcotic Depth (END). We need to calculate the loop fraction of helium first as this will then tell us what we need to have in our diluent. Remember that because we are on a constant PO2, our loop gas will not be the same as our diluent and in fact will contain a little more oxygen than our diluent. As we are considering oxygen to be narcotic, we need to take this slight increase in FO2 in to account.

So, the fraction of helium needed in the loop (FHe loop) =

1 - (END/P) where END and P (pressure at actual dive depth) are in absolute pressure.

If we chose a 30m END = 4 Bar, and have already planned our dive depth to be 70m, P = 8 Bar

1 - (4/8) = .50

So, the FHe in the loop is .50 (or 50% He)

In order to calculate how much helium we need in the diluent we simply use:-

FHe diluent = 1 - [(1 - FO2 loop - FHe loop) + FO2 dil)]

Firstly, let us check the FO2 in the loop.

Setpoint/P will give us this so:-

1.3/8 = .1625 (rounded down to .16 or 16%)

Therefore, using the above formula, the helium we want in our diluent cylinder is:-

FHe dil = 1 - [(1 - .16 - .50) + .12]

This leads to a diluent FHe of .54 or 54% in our diluent cylinder.

Some of you might have slightly different figures depending on if or how you have rounded up or down your calculations. I tend to round down the O2 and round up the He for conservatism.

Please bear in mind that none of this is a substitute for proper CCR and trimix training.

I really would like to say thank you to all those at this years Tek Camp who came to listen to my presentation. Unlike many of the other (excellent) presentations, mine did not include exciting wrecks or caves or big diving expeditions and was perhaps a little bit scientific. Nonetheless I had a lot of very positive feedback from a very interested audience. The talk focused on oxygen and some of it's effects on our physiology when diving. So, following on from a previous post looking at choosing OC gasses and including one of the subjects I covered at Tek Camp, let us have a quick look at the subject of whether oxygen should be considered as narcotic for the purposes of diving.

There has been some research done on the subject and I will try to summarise some of the keys points. I will also try to dig out some of the references and post them at some point so you can look at the complete documents if you want.

We all know that the site for the narcotic effect is the brain, specifically the cell wall of the cells within the brain. Given the same partial pressure of oxygen or nitrogen at this site they will both be equally narcotic as predicted by the Meyer Overton law relating to fat solubility and to some extent the molecular size and weight to narcotic potency.

This holds true for all substances exerting an anaesthetic effect. The only difference between anaesthesia with an inhaled modern medical agent and nitrogen narcosis is the partial pressure required for the effect. This is about 0.01 of a bar of isoflourane and about 15 bar for N2 to produce the same level of unconsciousness!

However although the brain is very well supplied with O2 it is also a big consumer of O2. The brain uses oxygen at a high rate and so the tissue partial pressure remains low when breathing air at 1 bar. In order to supply the brain with sufficient O2 we have the compound haemoglobin in blood cells which increases the amount of oxygen carried from 0.3 mls per 100mls dissolved in plasma to 20mls/100mls in red cells with haemoglobin. When breathing air the haemoglobin is fully saturated with oxygen in blood going to the brain in healthy individuals.

This means that increasing the PPO2 in the inspired gas has little effect on the amount of O2 delivered to the brain and so the tissue PO2 (our effect site for narcosis) does not rise until quite high PPO2s, probably in the region of 1.5 - 2.0 bar. This is also the region of CNS O2 toxicity and hence not often breathed underwater or if so, not usually for extended periods of time or at great depth.

Thus whilst O2 is narcotic and can be demonstrated to be so under controlled chamber conditions at a PPO2 of around 3.0 bar, the risk of CNS O2 toxicity also limits the narcotic effect in practical terms underwater.

Incidentally, the effect of increased CO2 (from skip breathing, poor breathing patterns or an issue with CO2 removal or component failure in a CCR) increases blood flow to the brain. CO2 is an epileptogenic, i.e. high levels of CO2 lower the threshold at which a seizure may occur. Increased CO2 is a far more effective way of increasing O2 flow to brain tissue and helps explains an increase in narcotic effect and heightened risk of CNS O2 toxicity and convulsions in the ‘borderline’ region of 1.5 - 2. 0 bar inspired PPO2.

So, both points of view are essentially correct in different circumstances.

where FN2 is the fraction of nitrogen you will have in your trimix, the END is the Equivalent Narcotic Depth you have chosen and P is the absolute pressure of the dive depth.

If we follow on from the example used in the previous post where we used a dive PO2 of 1.35 Bar to calculate our FO2 for a 60m dive we had an FO2 of .18 or 18%.

Our chosen EAD for that dive was 30m so for an END of 30m we use:-

END = 33m = 4.3 Bar

P = 60m = 7 Bar

FN2 = (4.3/7) - FO2

FN2 = (4.3/7) - .18

FN2 = .434

In the previous post our mix was calculated to be 18/37 (i.e. .FHe of 37 or 37% HE). Using the formula to include O2 as narcotic gives us a mix of 18/39, so just a little bit more helium in the mix when counting O2 as narcotic.

There are several reasons for taking the time to make sure we choose the right gas when diving on trimix. It allows us to ensure we are diving on a safe PO2 and to control our levels of narcosis.

When we do choose our OC gasses there is guidance on what might or might not be a safe or appropriate gas, so let’s just review some of the generally accepted OC practices for choosing trimix. At the moment we will just look at our deep gas. Intermediate and decompression gasses will be looked at in a separate post.

If we look at the ‘pressure T’ we can determine the best mix, MOD or TOD or safe depth for a particular gas.PO2 is the partial pressure of oxygen, FO2 is the fraction of oxygen in the gas mix and P is the depth expressed an an absolute pressure. Whatever we wish to determine, we simply ‘cover that up’ and then do the remaining sum.

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PO2 = FO2 x P (to determine the PO2 of a gas at a depth)FO2 = PO2/P (to determine the best mix for a gas at a a particular partial pressure and depth)P = PO2/FO2 (to determine the MOD or TOD of a gas at a particular partial pressure)

For example, the absolute pressure ‘P’ at 30 metres is 4 bar, Nitrox 32 or 32% has an FO2 of .32 and 32% at 30 metres will have a PO2 of 1.28 (.32 x 4 Bar).

When choosing a Nitrox for shallower diving the staring point is usually the PO2 of the gas at our Target Operating Depth (TOD) or Maximum Operating Depth (MOD), depending on the nature of the dive.When choosing a Trimix we would have the same start point but with some adjustments. So let’s quickly recap the ‘starting PO2s’ we might opt for.

A PO2 of 1.6 is, as we know the maximum limit. Normally used in technical diving rather than recreational and typically only for decompression gasses.

A PO2 of 1.5 is often called the recreational limit.

A PO2 of 1.4 is similarly called the working limit and very commonly used as the start point for calculating the FO2 for the best mix for a trimix dive. There are some other adjustments which you might choose to adopt, again depending on the nature and exposure even of the dive.

In it’s simplest form with no adjustments or safety factors applies an example for a 60m dive would look like this.

60m = 7 Bar absolute

PO2 of 1.4/7 = .20 (or 20% O2)

So we now have an FO2 for your trimix.

We all know that there are a number of factors which have the potential to make us more susceptible to CNS oxygen toxicity. I’m not going to review them all but included amongst them is getting cold, increased CO2 production and the length of exposure to the elevated PO2. Hard work underwater will increase our CO2 production. So for cold, a hard working dive or a long dive we can reduce our start point PO2 for calculating our FO2. The reduction factor is 0.25 for each.

From the example above we might be planning a relatively straight forward 60m dive as far as bottom time goes but water is a bit chilly cold and there might be a current flowing so we could choose to reduce ur starting PO2 by 0.25 for each of these factors. This would give us a starting PO2 for our calculation of 1.35.

1.35/7 = .193 rounded up so we might choose .18 or .19 (18% or 19% O2) as our FO2 in our trimix.

As an aside, it is fairly standard within the dive industry that when a dive centre mixes nitrox or trimix then a variation of + or - 1% is acceptable. It is better to err on the side of caution and ask for a slightly lower FO2 so that if it comes out a tiny bit rich it is still going to be a safe gas.

We can, if wanted, apply a ‘safety factor to our decompression gasses too, of -0.05 Bar PO2 for each of cold or long dive time (there shouldn’t really be any hard work on deco!) This is an additional safety margin we can put in to control our CNS% and OTU loading on a longer dive.

Having chosen our FO2 we then need to calculate how much Helium we want in order to control our levels of narcosis. We perhaps should set aside for another debate the question of O2 being narcotic. Yes, the mathematics and physical properties of the gas suggest it should be but there are equally compelling agreements, based on scientific investigations, that suggest in real diving, practical terms, it is unlikely to actually cause narcosis. If I can dig out some research I can post a synopsis of some of the theories. Based on these documents I personally fall on the side of not taking in to account the potential for O2 to be narcotic so I will leave that out of the process of calculating the He to add in our mix.

The start point should be your own comfort levels as far as narcosis is concerned. Some are happy to dive to a deeper level of narcosis than others so it really can be a very person choice but we should try to keep some level of team compatibility if we can.

So let us choose 30 metres as our equivalent air depth (EAD). We need to calculate what the partial pressure of nitrogen (PN2) is at a depth of 30m. So, the absolute pressure at 30m is 4 Bar, so the PN2 is 4 x .79 = 3.16.

3.16 is the peak PN2 we are aiming for at our actual dive depth of 60m. If we divide 3.16 by the absolute pressure at 60m of 7 Bar (3.16/7 = 0.451) we get an FN2 of 0.45. This is how much N2 we will allow in the mix.

To calculate the FHe we simply subtract the FO2 and FN2 from the whole and what is left is the FHe.

So, 1 - .18 - .45 = .37. Our FHe in our mix is .37 (37%). Our gas mix is now calculate as Trimix 18/37. (It is common to give the FO2 first then the FHe when deciding a trimix gas).

So, all fairly straight forward. Bear in mind that this process is used for OC gas calculations. We would adopt a slightly different approach and apply slightly different rules when calculate a trimix for use in a CCR both for onboard active diluent and bailouts.

Click on this Youtube link below to check out the video. I was asked to help out with the filming of the latest promo video for cave diving in the park. We will be back there in a few weeks (sadly no spaces left on that course) and again in early December (we do have one space left for that cave course. It can be for OC Tech Cave or CCR Cave.https://youtu.be/7d5ugp74flQ

GasMatchingGas matching is an important part of the dive planning process and an essential component of any pre-dive safety checks. If you and your team have a bad day and end up having to share air it could make the difference between a comfortable swim out of the cave and a rather frantic race, worrying that your gas reserves might not last. Gas matching is nothing more complicated than adjusting the size of your reserve to compensate for factors that might affect it. We are basically making sure that the reserves we plan are actually large enough to support our exit in an air sharing emergency.

So, when do we gas match? I am sure others could add to the list but here are a few examples for guidance.

Any time when in a team of two divers, the diver with the largest RMV is also using the largest cylinders.

Anytime when the team has to negotiate restrictions.

Anytime, even in a team of three or more, where there is a breathing rate difference of more than double between two divers.

When don’t we need to gas match?

In a team of three or more divers (apart from the breathing difference mentioned above)

A team of two divers using the same sized cylinders (exception as above)

What does all this mean? Let’s look at them more closely.

If a diver with a larger breathing rate uses larger cylinders (which, let’s admit, is quite common) then if something goes wrong at the point of furthest penetration and it is the bigger breather who needs to support the smaller breather then his larger cylinders plus the smaller breather should mean ample gas to swim home. If it is the other way around however, then it is quite likely that the amount of gas in reserve in the smaller breather’s smaller cylinders might not support both of them out of the cave.

When negotiating restrictions with all other things being equal, it should take the same time to travel in through the restriction as it does out. However a considerable amount of extra time might be needed when divers are sharing air through a restriction, hence the need to check that the reserves are going to be big enough.

If you have go someone on the team with a very large SAC then it would be a good idea to check the numbers to ensure the reserve volume is sufficient to support them out of the cave if you have to donate gas.

For my imperial friends we have the delights of what are known as ‘dissimilar tank calculations’. I am going to leave that for a separate short article. Those who have to use these will no doubt understand!

How do we do the maths for gas matching. One way is to cheat a little and get a gas matching table. IANTD certainly have them and they are easy to use.

Another way is to do some simple maths just to make sure the reserves are large enough. It mainly boils down to adjusting your turn pressure, turning sooner than would be expected, using less gas on the way in and out and keeping more than 1/3 in reserve.

As an example if my buddy has an SAC of 24 litres per minute and mine is lower at 12 lpm I can look at a chart at mine and my buddies SAC rates and quickly read the SAC Ration Factor (SRF). The SRF is nothing fancier than a new percentage of my starting pressure at which I need to turn. The usual 1/3 is roughly 66%. Looking at the SRF for mine and my buddies SACs I need to turn earlier at 76%. So for a start pressure of 230 bar I will turn at about 175 bar, thus making sure my reserve third is big enough to support my buddy if needed. I didn’t do the maths, I just flipped the chart over to check the numbers where my SRF crosses my start pressure.

If you dive with the same team then it is easy to pop the numbers down in your wet-notes, either from the a chart or from sitting down with a calculator!

Why is this important? Well if you don’t match gas, this could happen.

Diver A has an SAC of 12 litres per minute and is diving on twin 12s charged to 200 bar

Diver B has an SAC of 15 litres per minute and is diving on twin 15s charged to 200 bar.

They will reach their turn pressure at the same time. If, at that moment Diver B has a catastrophic gas loss and needs to share gas with Diver A…..well Diver B will have used 2000 litres of gas going in. Diver A will have used 1600 litres. Diver A will need the same volume to get out, 1600 litres leaving his 1/3 in reserve at 1600 litres. Diver B still needs 2000 litres of gas so that is a 400 litre shortfall. This isn’t going to have a happy ending.

If on the other hand the dive team match gas, and calculate that Diver A needs a reserve big enough to match the gas volume needed by his buddy…..well Diver A just needs 2000 litres as a reserve, leaving his usable gas at 2800 litres, 1400 litres for the dive in, 1400 litres for the dive home. Depending on the depth of the dive that extra 200 litres from each leg of the journey now put aside for the reserve might only be just a few minutes further in but puts the gas plan back in to the realms of being safety first.

1400 litres from the starting pressure means Diver A turning the dive at 145 bar rather than 135 bar.

All of this has been simplified slightly so it does not take in to account deeper cave diving with a significant decompression obligation or any other multi-stage extended range penetration dive. That is probably better left to the classroom as part of a Deep Cave Diver or Multi-Stage Cave Diver course but the main principles are very similar in concept.

I consider myself to be very lucky to have the opportunity to regularly visit Florida to teach Cave Diver classes. My trips are often a few months apart and so I notice changes as soon as I see them. It might be different if I dived the caves every day or every week. Familiarity might make me miss subtle changes until they become more profound.

Over the last couple of years one of the changes (and not for the better) that has become very noticeable is the standard of line laying. Now, don't get me wrong, this article is not trying to point fingers and say 'he or she did this wrong', but just to discuss why this might be the case, what the problems are and maybe even offer some pointers that might help the next time you lay a line.As a Cave Instructor Trainer not only do I want to ensure that cave students get well rounded training but also that new Cave Instructor candidates continue to pass on these skills and their importance.

Years ago I was a Trainer in a Fire Department. It might surprise you to know that, at least in the UK, fire-fighters use guidelines! They are a bit thicker than cave lines and have tactile markings on them, indicating direction, in or out, which can be felt whilst wearing thick fire-fighting gloves but they are deployed to help Fire-fighters safely enter, search and exit a smoke filled building. They are not necessarily used in smaller domestic properties but in larger commercial and industrial premises. I am sure that we can all understand the hazards of getting lost in a smoke-filled building that is on fire. I used to teach the procedures and protocols for laying these guidelines and the important role they play in Fire-fighter safety. I stress the same things to my cave diving students and perhaps this background is the origin of why I feel that line laying is such an important part of a cave diver's skills. So, back to the caves. It would be difficult to accurately pin-point a particular reason for the decline in line laying skills but I think there are a few factors that have contributed. I am not going to say whether I think any of these are good or bad, but just highlight some changes that have had an impact on this particular skill.

The first, and to me most obvious, is that in a number of caves some divers believe we don't strictly need to run our own primary line in to the cave to get to the permanent line. A number of cave sites have seen the 'gold line' (a term used a lot in Florida for the permanent main line in a cave due to their colour) move progressively closer and closer to the cave entrance. Whilst they may not be quite out to open water, their proximity does mean that fewer and fewer divers bother to run their own primary line. Whether we like this or not it does mean, for most cave divers, less practice aying lines.

Next, there are more participants in the sport. More cave divers means busier systems and regardless of where a permanent line might be inside the cave, a great many cave divers choose not to lay their own line. This decision might be because there does not appear to be enough space to lay their own, or they think they know the cave well enough not to need one, or they want to leave space for others to lay a line, for example a team of experienced divers choose not to lay a line in order to leave space for an Instructor with students to lay a line.

Dare I say it, but Instructors might also be contributing toward the decline of line laying skills. Lines nearer to open water means that students might only have to lay a few feet of line to the 'gold line’. Instructors don't need to take the time to teach good line laying skills and can get their students further back much quicker and so the students get more 'instant gratification' of longer penetrations on their cave course without the boring bit of having to lay a long and difficult line

So why lay a line if we don't think we need to? The golden rules of cave diving are as relevant today as they always have been. 'Always maintain a continuous guideline to open water/the surface/a safe area (choose which ever you think is best) is not just an old adage. Choosing any one of these, preferably one of the first two options, so that there is little room for confusion, sounds like a good idea to me, even if you think you know the cave really well.

I recall exiting from the upstream side of Cow Spring one sunny afternoon to see another diver very obviously 'lost' and somewhat stuck where he had taken a wrong turn, just a few feet from open water and found himself in amongst the boulders from where he couldn't figure out an exit route....

Just a few weeks ago,whilst cave diving in France, a solo diver swam past us on his way out of the cave. He was less than 10m/30ft from being in the head pool and that included 6m/20ft of vertical ascent through a boulder chimney. As he swam past us (we were busy assembling our under water habitat at the time) he gave us a cheery wave and then looked up, banged his head on the ceiling and realised he couldn't remember his route through the boulders to open water. We were filming our 'construction site' at the time and the look on his face, captured on video, is an absolute picture! Needless to say we pointed to our line and exit route and he went happily on his way. (I did the same for the chap a bit stuck in Cow Spring, so happy endings all round).

I can't think of a better reason to have that bit of string in place, even if it's just for piece of mind. You might think know your way around really well but do the rest of your team? Are they happy that they don't have a primary line to the outside world? It only takes a few minutes.

So why else? It does also let other divers know roughly in which direction you have headed. A god-send if you are hoping someone might come looking should you get a little lost further back. From an Instructor's view point, it does the same. I know there are divers ahead of me and if I am planning some drills, skills and fun and games I can go somewhere else so that I don't interfere with their pleasure dive. If I do head your way then I will have an expectation of meeting another team and can use this as a learning exercise for my students.

Is there anything you can do to improve your line laying skill? Well for a start, if you see me in North Central Florida, France or anywhere else for that matter and want some hints or tips, just ask. Or even jump in on a dive with me. Just come and ask, you would be very welcome. There is always a fair amount of line laying done on most of my teaching dives. I am pretty sure that there are a number of other Instructors who would do the same.

On top of that, when you do lay your line, just stop for a second and look into the cave to see where (if there are any) the other lines have been laid. This gives you the chance to pick the best spot for you own primary tie-off. Be honest, how many times have you made a primary then secondary tie-off, then headed in to the cave and thought **%%*^&, I wish I had tied off on this/that side instead! (Of course we always go back out and start again, don't we?)

Try to remember some of the rules and protocols that your Instructor taught you. Trying, where possible, not to parallel another line too closely, not using the same tie-off point as another line, not using another line as a tie-off point(!) I have this seen quite a few times recently– nowhere to tie-off, just make a few tie-offs on other lines in the cave!. I also recently saw a line zig-zagging through the cavern zone at Peacock 1 tied alternatively between points on the main line and the opposite side of the cave! Not only do poor lines like these cause potential entanglements but put stress on the gold line and make it very difficult for other divers to lay their lines! The only time we should be tying our line onto another is to secure our spool or reel onto the line we are now going to follow.

Keep your line tight and out of the way of other divers' lines. This will make it easier for you when you turn and reel out of the cave, and will help to keep the others away from your own line, making it less likely that any of your tie-off points will be accidentally kicked or pulled off.

Take your time and look ahead. Try and plan the route you will take and look for possible tie-off points well in advance. This will also make buoyancy control whilst tying-off much easier as you will have anticipated your actions.

Don't forget the team work aspect. The second divers should be helping to illuminate possible tie off points then lighting the one you choose if the cave configuration allows and checking for line traps.

Safety should be the paramount thought for any cave diver. I would encourage all of us to think about this before you decide whether or not to lay a line, be it a primary line or a jump or gap. Just for a second also think about the signals you send out to newer cave divers. 'Our Instructor made us lay a line all the time, but once you get a bit more experienced you don't need to, because, hey, take a look, none of these guys do and they are really experienced'.

Rule of ThirdsAsk any group of tec divers what is the rule of thirds and you will probably get a number of different answers. In all probability they will more than likely be close in concept and close to the actual rule but with a few variations in wording. But what do they actually mean?

“1/3 in, 1/3 out and a 3rd in reserve”, “surface with a third of all your gas”, “save a third of your gas for emergencies” are typical statements. You might also hear divers say that they dive ‘to’ the rule of thirds but is that going to be the conservative option if you do need to utilise your reserve gas supply?

The rule of thirds is variously attributed to either the pioneers of cave diving in the UK and the founders of what was to become the Cave Diving Group (CDG) or to their slightly more modern counterparts exploring the springs of North Central Florida.

Regardless of origin, the rule was used by cave divers, allowing one third of the gas supply to be used on the inward journey, one third for the dive out and a third held in reserve in case things did not go according to plan.

In simple terms it seems to work. Looked at more closely there are flaws, albeit only minor, which need to be addressed as part of your dive plan in order to stay safe if things go wrong.

First let us look at the common belief that we should dive ‘to’ the rule of thirds. For this example we will assume identical breathing rates and identical cylinder sizes between two dive buddies. Should one diver have a catastrophic loss of gas at the furthest point of penetration the other diver has a third of his gas ready to donate to the out of air diver. However if the process of the first diver trying to deal with the gas lost in the first instance takes any time or the act of sharing air and sorting themselves out for the exit takes more time, the diver donating gas will have been using some of his or her exit gas already. Those few minutes to get organised for the swim out could leave gas supplies perilously close to the limit or even being exhausted prior to the team reaching safety.

What if the air sharing swim out of the cave takes much longer just because the long hose has been deployed? What if the two divers did not have identical breathing rates? If the out of air diver has a measurably larger breathing rate then the donor diver’s reserve third might simply not be enough gas!

How can we plan to avoid these potential pitfalls? Firstly we can look at how we might apply the rule of thirds before we discuss other planning considerations. When I said that many divers will dive to the rule of thirds the implication is that they will turn the dive when the hit the point of having used 1/3 of their gas. But turning around takes time. Making sure everyone on the team has seen the signal and they too have turned takes time. All this is eating in to the gas for the swim out and as so in to your reserves as well. So, why don’t we look ahead. Rather than wait until you reach the exact turn pressure, consider turning just a few bar/psi earlier. If your turn pressure is 140 bar consider turing when you are at 145 bar. (For my non-metric friends a similar example might be a turn pressure of 2000 psi so perhaps turn at 2100 psi). Let’s be honest, we aren’t really going to get that much further in for another 5 bar and we can always come back another day with more gas!

Think too about where you might turn the dive. Right now might be a good time because if you swim another minute or so you and the team could be in a smaller cave passage making turning more difficult and if you keep going to where it opens out again it is almost certain you will have gone past your gas turn pressure.

So, rather than diving to the limit of the rule of thirds, we should try to dive within it.

Perhaps now would be a good time to have a look at how we go about calculating our turn pressure based on the rule of thirds.

Let’s say we have a starting pressure of 210 bar. That one is nice and easy, 210 divided by 3 is 70 bar, so each third is 70 bar. 70 bar from our start pressure would mean the turn pressure is 140 bar.

(A simple imperial example would be a starting pressure of 3000psi. Again, nice and easy, 3000 divided by 3 is 1000psi, so each third is 1000psi. 1000psi from our start pressure would mean the turn pressure is 2000psi).

What if the stat pressure is not a nice easy number to divide by 3? I know a few divers who can seemingly instantly divide almost any starting pressure by 3 and calculate their turn pressure. I have also seen diver then use this very precise pressure during the dive when they have digital readings or pressure. However I have also noticed how this seems to entice those divers to go right to the limit of their gas.

Simpler and safer would be to err on the side of caution and make the maths easier. Just for fun I will look at a couple of examples.

A starting pressure of 220 bar is not easy to divide by three in your head whilst floating in the head pool of a cave discussing the dive plan and turn pressures. Far easier is to round the number down to the next lowest number that is easily divisible by 3. In this case that would be 210 bar. The usual turn pressure for 210 bar would be 140 bar (210 - 70) but in this case we are starting with 10 bar more, so very simply, calculate how big 1/3 is from our rounded down pressure (70 bar here) and subtract that from your actual starting pressure. 220 bar - 70 bar would give a turn pressure of 150 bar. 70 bar in and 70 bar out would leave the ‘largest 1/3’ as the reserve, 80 bar.

What about 200 bar start pressure. Well 195 bar is easily divisible by 3. Each 1/3 would be 65 bar. 65 bar subtracted from your actual start pressure would give a turn pressure of 135 bar.

For some that calculation might not be so easy but there is no reason why you can’t write a list of start pressures and turn pressures in your wet-notes so you don’t have to worry that your maths isn’t so good or that you might get it wrong.

When you have calculated or looked up your turn pressure don’t just tell the team what your turn pressure is, tell them how much gas you have and then what your turn pressure will be. That way you get a few other brains to check that you have got it right. After all, it is their safety too that could be jeopardised if someone gets it all wrong.

Remember too that for wreck penetration dives, while still using the same approach with the rule of thirds, it is applied a little differently. We have to take in to account the fact that having exited the wreck, in most cases, we still have an ascent to make and possibly decompression too. The same might apply to a very deep cave dive. Perhaps that will be for a another post in the future.

What else can we do to to add conservatism and safety to how we use this rule?

There is something called gas matching which we can apply to make sure we don’t get caught short on our gas supplies.